Gap Regeneration Research Articles

Diversity of Tree Species Regeneration in Natural Gaps of Kafa Moist Afromontane Forest, South West Ethiopia Peoples` Region, Ethiopia

Diversity of Tree Species Regeneration in Natural Gaps of Kafa Moist Afromontane Forest, South West Ethiopia Peoples` Region, Ethiopia

Liana regeneration in the treefall gaps, at the gap edges and in the forest interior of a subtropical montane moist evergreen broad-leaved forest in the Ailao Mountains, Southwestern China

Liana regeneration in the treefall gaps, at the gap edges and in the forest interior of a subtropical montane moist evergreen broad-leaved forest in the Ailao Mountains, Southwestern China

Effect of gap size on the regeneration in Larix principis-rupprechtii plantation.

We conducted a survey on seedlings (height <1 m) and saplings (height ≥1 m, diameter at breast height <5 cm) in 20 gaps of Larix principis-rupprechtii plantations on Guandi Mountains, Shanxi to analyze regene-ration density, growth indicators, and spatial distribution of L. principis-rupprechtii seedlings and saplings under four gap sizes (<60 m2, 60-120 m2, 120-180 m2, and ≥180 m2). The results showed that growth indicators (ground diameter, height) of seedlings and saplings and regeneration density of seedlings were highest in small gaps (14-60 m2). The sapling regeneration density was highest in medium gaps (60-120 m2), and sapling density exceeded seedling density in each size category. L. principis-rupprechtii seedlings and saplings exhibited favorable regeneration in small and medium gaps, while large gaps (120-180 m2) and extra-large gaps (≥180 m2) were unfavorable for L. principis-rupprechtii regeneration. L. principis-rupprechtii seedlings and saplings were mainly distributed within the canopy projection area and along the edge of canopy gap area. Controlling gap size within the range of 14-120 m2 through artificial interventions, such as planting and thinning, could promote the regeneration of L. principis-rupprechtii.

A forest gap is not forever: Towards an objective standard to determine when a gap is considered closed in temperate forests

Forest gaps create environmental heterogeneity and drive forest dynamics through gap-phase regeneration. However, it is unclear when a forest gap can be considered closed, limiting a global synthesis of forest succession that integrates gap patterns and dynamics. We created twelve forest gaps (six large gaps, RD/H [ratio of gap diameter to gap border tree height] = 1.3; six small gaps, RD/H = 0.6) and monitored tree regeneration for seventeen years to establish an objective gap-closure standard for regeneration filling in a temperate secondary forest. Our results indicated that a forest gap could be considered closed or entering a gap-closure phase when the dominant gap regeneration layer (i.e., the top 10% gap-fillers) (1) reached the crown base height (8.9 m on average) of canopy trees and (2) entered a period of stability in density, richness, and evenness. Linear mixed-effects models found that these two conditions met each other in the same period (14.5 years) in the large gaps, with a mean density, richness, and evenness of 0.1 (stem m−2), 4.9 (species gap−1), and 0.9, respectively. However, if no repeated gap disturbance occurred, small gaps were closed by lateral extension rather than by height growth. Light-demanding species (e.g., Juglans mandshurica) mainly contributed to the closure of large gaps and played negligible roles in the small gaps, but intermediate or shade-tolerant species (e.g., Acer mono) in the small gaps had potential for successful gap filling following repeated gap disturbance. Our gap-closure standard links gap structure to species composition and provides a reference for large-scale research on gap patterns and dynamics, which helps to understand the ecological process during forest succession. The standard also provides a relatively clear boundary between forest gaps and closed forest stands, which is a premise to develop appropriate forest management and shorten the restoration period of degraded secondary forests.

Disentangling regeneration by vertical stratification: A 17-year gap-filling process in a temperate secondary forest

Forest gaps are widely spread in the forest ecosystems and play important roles in driving forest dynamics. However, long-term regeneration during the gap-filling process was less understood, especially species composition and dynamics along vertical gradients over time. We monitored tree regeneration in artificial gaps of a temperate secondary forest for 17 years and analyzed the gap-filling process both as a whole and by stratification (lower layer: 0∼3 m, middle layer: 3∼6 m, and upper layer: >6 m) for three shade-tolerance groups of tree species. We found that the interaction effects of gap size and shade tolerance could result in a growth difference of five to ten years among different regeneration groups, with the most significant effects in the large gaps for the light-demanding species such as Juglans mandshurica and Phellodendron amurense. Regeneration density in the large and small gaps eventually converged, but there were great differences in density dynamics. The regeneration density in the large gaps decreased gradually over time, while it decreased sharply in the small gaps after just one growing season of gap formation and consistently remained at low levels regardless of shade tolerance. Species richness decreased smoothly during the 17-year monitoring, but the richness in the large gaps was consistently higher than that in the small gaps, with all shade-tolerance groups existing, which underlined the importance of gap size in maintaining tree species diversity. Differences in the density, richness, and composition among regeneration layers reduced over time. However, vertical regeneration patterns diverged among shade-tolerance groups during the gap-filling process. Light-demanding species were likely to have only one chance for successful regeneration because almost no new recruitments appeared in the lower or middle layer as dominant stems reached the upper layer. Intermediate and shade-tolerant species, by contrast, were well-distributed in different regeneration layers, indicating a stronger capacity for resistance and resilience during uncertain disturbance events in the long run. Our results revealed the gap-filling strategies of tree species with different shade-tolerance groups, which explained why light-demanding species could not completely occupy a forest gap and highlighted the necessity of repeated gap disturbance for intermediate and shade-tolerant species regeneration.

Influence of Earlier Snowmelt on the Seedling Growth of Six Subboreal Tree Species in the Spring

Climate warming is advancing snowmelt timing in the spring at high latitudes. To predict tree growth in subboreal forests under warmer climates based on mechanistic understanding, it is important to assess how advancing snowmelt influences tree growth in the spring via ecophysiological changes in subboreal forests. In this study, we conducted a field manipulation experiment of snowmelt timing and investigated the response of tree growth, leaf functional traits, and bud-burst phenology in the spring for the seedlings of six dominant tree species in subboreal forests. We found that the spring growth of only one species (Kalopanax septemlobus) out of six species responded positively to advancing snowmelt. Among the leaf functional traits (leaf mass per area, leaf nitrogen content, leaf δ13C value, leaf dry matter content, and leaf area) and bud-burst phenology, only the increase in leaf area was linked to the enhanced shoot growth of K. septemlobus. The significant change in K. septemlobus might be associated with its ecological characteristics to prefer regeneration in canopy gaps. These results indicate that advancing snowmelt under warmer winters can be beneficial for tree species that can plastically develop leaf area in Japanese subboreal forests.

Gap dynamics and disease-causing invasive species drive the development of an old-growth forest over 250 years

Investigating the disturbance regimes of unharvested forests helps us understand their past, present, and future trajectory and gives us a model for forest management. It can also clarify the relative importance of small-scale gap dynamics versus more severe disturbances. Here we used tree rings to examine the recruitment patterns, growth dynamics, and disturbance chronologies of American beech (Fagus grandifolia), sugar maple (Acer saccharum), and eastern hemlock (Tsuga canadensis) in A.B. Williams Woods, an old-growth forest in Ohio, USA, over the past 250 years. We found that beech and sugar maple recruitment peaked around 1900 and continued through the 1900s, while hemlock recruitment peaked during 1825–1875, then declined and effectively ended in the early 1900s. Hemlock grew fastest during the 1800s according to ring width and basal area increment, while sugar maple ring width surpassed beech and hemlock in the 1900s. All three species showed a dramatic increase in growth from 1980 to 2010. Beech and sugar maple established regardless of canopy gaps, but 73% of hemlocks originated in gaps. In most decades, <10% of trees experienced gap recruitment or growth release, suggesting that ongoing, endogenous canopy mortality was the primary disturbance shaping this forest. However, a more severe forest-wide disturbance occurred during the 1980s–1990s when the scale insect causing beech bark disease was introduced, with greater than 30% of living trees showing growth releases in those decades. Another synchronous release occurred in the 1930s when blight-killed chestnuts were removed; 16% of trees showed releases. Both of these intermediate-severity disturbances involved human introduction of invasive species. Thus, we documented a natural disturbance regime of small-scale gap dynamics, punctuated by more severe anthropogenic disturbances in the twentieth century. These relatively frequent, intermediate-severity events probably mean that the forest’s current composition is non-equilibrial. Hemlock may continue to decline while beech maintains its dominance by constant regeneration in both gaps and shade, and by responding to disturbance with root suckering and growth pulses. The codominance of sugar maple may be relatively recent, and perhaps temporary, as we found little sugar maple recruitment before 1875 or after 1950, and three times fewer sugar maple saplings now than in the early 1900s. Despite being protected as a park, the development of this old-growth forest has been shaped more by disease-causing invasive species than natural disturbances over the past century. This result emphasizes the pervasiveness of human impacts even in communities we look to as examples of natural pattern and process.

Graphene-Based Materials Prove to Be a Promising Candidate for Nerve Regeneration Following Peripheral Nerve Injury.

Peripheral nerve injury is a common medical condition that has a great impact on patient quality of life. Currently, surgical management is considered to be a gold standard first-line treatment; however, is often not successful and requires further surgical procedures. Commercially available FDA- and CE-approved decellularized nerve conduits offer considerable benefits to patients suffering from a completely transected nerve but they fail to support neural regeneration in gaps > 30 mm. To address this unmet clinical need, current research is focused on biomaterial-based therapies to regenerate dysfunctional neural tissues, specifically damaged peripheral nerve, and spinal cord. Recently, attention has been paid to the capability of graphene-based materials (GBMs) to develop bifunctional scaffolds for promoting nerve regeneration, often via supporting enhanced neural differentiation. The unique features of GBMs have been applied to fabricate an electroactive conductive surface in order to direct stem cells and improve neural proliferation and differentiation. The use of GBMs for nerve tissue engineering (NTE) is considered an emerging technology bringing hope to peripheral nerve injury repair, with some products already in preclinical stages. This review assesses the last six years of research in the field of GBMs application in NTE, focusing on the fabrication and effects of GBMs for neurogenesis in various scaffold forms, including electrospun fibres, films, hydrogels, foams, 3D printing, and bioprinting.

Spatial patterns and species coexistence in mixed Abies marocana-Cedrus atlantica forest in Talassemtane National Park

Understanding spatially-explicit interactions between co-existing tree species provides valuable information about the trade-offs between facilitation and competition driving species coexistence and forest dynamics. Here, we analyzed the fine scale spatial pattern of Moroccan fir (Abies marocana) and Cedrus atlantica (Cedrus atlantica) in a mixed forest (Talassemtane National Park, northern Morocco) and discuss its consequences on intra- and inter-specific interactions and stand dynamics. All trees were mapped and measured in ten plots to investigate uni- and bivariate spatial point patterns using pair-correlation and mark correlation functions. Tree ring data was also investigated from increment cores to assess the growth and age patterns of both species. Fir and cedar trees showed similar basal area and mean diameter distributions, while A. marocana presented a higher density and regeneration. There was an overall aggregation of conspecific and heterospecific individuals at small distances up to 2 m. Univariate analysis of tree size classes supports that juveniles of both species, as well as A. marocana adult trees, show an aggregated pattern at fine scales up to 2 m, while C. atlantica adults exhibit a random pattern. Observed spatial patterns are consistent with the hypothesis of attraction mechanisms between fir and adult trees of C. atlantica. Abies marocana recruitment mainly occurred inside the canopy of dominant trees (at distances varying from c.a 1 m under cedar adults to 3 m under adult conspecifics), while C. atlantica was the dominant regeneration in small gaps. The studied A. marocana-C. atlantica mixed forests showed a transition from an initiation phase led by C. atlantica to a subsequent stem exclusion phase, where dominant trees shade out less-competitive trees and mainly fir trees can establish in the forest understory. According to the observed spatial patterns and growth rates, enhancing C. atlantica establishment in degraded areas may be a suitable proactive and adaptive management to promote long-term conservation of endemic Abies marocana under current climate change trends.

Application of Human Epineural Conduit Supported with Human Mesenchymal Stem Cells as a Novel Therapy for Enhancement of Nerve Gap Regeneration

Various therapeutic methods have been suggested to enhance nerve regeneration. In this study, we propose a novel approach for enhancement of nerve gap regeneration by applying human epineural conduit (hEC) supported with human mesenchymal stem cells (hMSC), as an alternative to autograft repair. Restoration of 20 mm sciatic nerve defect with hEC created from human sciatic nerve supported with hMSC was tested in 4 experimental groups (n = 6 each) in the athymic nude rat model (Crl:NIH-Foxn1rnu): 1 - No repair control, 2 - Autograft control, 3 - Matched diameter hEC filled with 1 mL saline, 4 - Matched diameter hEC supported with 3 × 106 hMSC. Assessments included: functional tests: toe-spread and pinprick, regeneration assessment by immunofluorescence staining: HLA-1, HLA-DR, NGF, GFAP, Laminin B, S-100, VEGF, vWF and PKH26 labeling; histomorphometric analysis of myelin thickness, axonal density, fiber diameter and myelinated nerve fibers percentage; Gastrocnemius Muscle Index (GMI) and muscle fiber area ratio. Best sensory and motor function recovery, as well as GMI and muscle fiber area ratio, were observed in the autograft group, and were comparable to the hEC with hMSC group (p = 0.038). Significant improvements of myelin thickness (p = 0.003), fiber diameter (p = 0.0296), and percentage of myelinated fibers (p < 0.0001) were detected in hEC group supported with hMSC compared to hEC with saline controls. At 12-weeks after nerve gap repair, hEC combined with hMSC revealed increased expression of neurotrophic and proangiogenic factors, which corresponded with improvement of function comparable with the autograft control. Application of our novel hEC supported with hMSC provides a potential alternative to the autograft nerve repair.Graphical

Sugar maple (Acer saccharum) Seedling Bank Response to Storm Disturbance and Single Tree Selection Harvest in the Southern Keweenaw Peninsula, Michigan

This case study seeks to fill a critical knowledge gap regarding how natural wind disturbance affects stand and seedling bank diversity in mixed northern hardwood forests managed by single tree selection harvest methods. Contemporary timber harvests on state managed lands in Michigan's Upper Peninsula employ single tree selection cutting methods to promote more complex age structure in second growth stands dominated by sugar maple (Acer saccharum). However, concern exists that single tree selection harvest may result in lowered compositional diversity of seedlings as the low light conditions of small dispersed gaps exclude less shade tolerant species from gap regeneration. In July 2016 severe thunderstorms with winds in excess of 145 km/h (90 mph) caused extensive tree fall and canopy gap creation in second growth mixed northern hardwood forests in the southern Keweenaw Peninsula. We measured the species composition of overstory tree mortality and understory seedling regeneration in 14 storm gaps created in stands with and without previous single tree selection harvest. Storm gaps ranged in size from 125 to 1100 m2. American basswood (Tilia americana) was disproportionately wind-thrown. Robust seedling regeneration was released in all storm gaps, with sugar maple comprising more than 75% of mean seedling abundance, regardless of previous single tree selection harvest. Sugar maple and ironwood (Ostrya virginiana) comprised 80% of sapling abundance. Results indicate single tree selection of mixed northern hardwood stands does not exacerbate, but rather emulates, dense sugar maple regeneration found on unmanaged second growth sites.

Differential spatial responses of rodents to masting on forest sites with differing disturbance history.

Mast seeding, the synchronized interannual variation in seed production of trees, is a well‐known bottom‐up driver for population densities of granivorous forest rodents. Such demographic effects also affect habitat preferences of the animals: After large seed production events, reduced habitat selectivity can lead to spillover from forest patches into adjacent alpine meadows or clear‐cuts, as has been reported for human‐impacted forests. In unmanaged, primeval forests, however, gaps created by natural disturbances are typical elements, yet it is unclear whether the same spillover dynamics occur under natural conditions. To determine whether annual variation in seed production drives spillover effects in naturally formed gaps, we used 14 years of small mammal trapping data combined with seed trap data to estimate population densities of Apodemus spp. mice and bank voles (Myodes glareolus) on 5 forest sites with differing disturbance history. The study sites, located in a forest dominated by European beech (Fagus sylvatica), Norway spruce (Picea abies), and silver fir (Abies alba), consisted of two primeval forest sites with small canopy gaps, two sites with larger gaps (after an avalanche event and a windthrow event), and a managed forest stand with closed canopy as a control. Hierarchical Bayesian N‐mixture models revealed a strong influence of seed rain on small rodent abundance, which were site‐specific for M. glareolus but not for Apodemus spp. Following years of moderate or low seed crop, M. glareolus avoided open habitat patches but colonized those habitats in large numbers after full mast events, suggesting that spillover events also occur in unmanaged forests, but not in all small rodents. The species‐ and site‐specific characteristics of local density responding to food availability have potentially long‐lasting effects on forest gap regeneration dynamics and should be addressed in future studies.

Tree mortality and regeneration of Euphrates poplar riparian forests along the Tarim River, Northwest China

BackgroundTree mortality and regeneration (seedling and sapling recruitment) are essential components of forest dynamics in arid regions, especially where subjected to serious eco-hydrological problems. In recent decades, the mortality of the Euphrates poplar (Populus euphratica) along the Tarim River in Northwest China has increased. However, few studies have quantified the causes of mortality and regeneration in this azonal riparian forest type.MethodsThe present study describes the annual hydrological response of tree mortality and regeneration in forest gaps. A total of 60 canopy gaps were investigated in six replicate grid plots (50 m × 50 m) and the annual runoff and water consumption data during the period of 1955–2016 were collected from hydrological stations in the middle reaches of the Tarim River. We compared the regeneration density of seedlings and saplings within the canopy gap areas (CGAs), undercanopy areas (UCAs), and uncovered riverbank areas (RBAs) through detailed field investigation.ResultsOur study found that the mortality of young and middle-aged gap makers has increased remarkably over recent decades, particularly since the year 1996. The main results indicated that regional water scarcity was the primary limiting factor for long-term changes in tree mortality, as shown by a significant correlation between the diameter at breast height (DBH) of dead trees and the annual surface water. The average density (or regeneration rate) of seedlings and saplings was highest in the RBAs, intermediate in the CGAs, and lowest in the UCAs. Compared with the UCAs, the CGAs promote tree regeneration to some extent by providing favorable conditions for the survival and growth of seedlings and saplings, which would otherwise be suppressed in the understory. Furthermore, although the density of seedlings and saplings in the CGAs was not as high as in the RBAs, the survival rate was higher in the CGAs than in the RBAs.ConclusionForest canopy gaps in floodplain areas can play a decisive role in the long-term germination and regeneration of plant species. However, as a typical phreatophyte in this hyper-arid region, the ecosystem structure, functions and services of this fragile P. euphratica floodplain forests are threatened by a continuous decrease of water resources, due to excessive water use for agricultural irrigation, which has resulted in a severe reduction of intact poplar forests. Furthermore, the survival of seedlings and saplings is influenced by light availability and soil water at the regional scale. Our findings suggest that policymakers may need to reconsider the restoration and regeneration measures implemented in riparian P. euphratica forests to improve flood water efficiency and create canopy gaps. Our results provide with valuable reference information for the conservation and sustainable development of floodplain forest ecosystems.

Gap regeneration and dynamics: the case study of mixed forests at Křtiny in the Czech Republic

Abstract Gap regeneration remains the best silviculture technique for sustainable forest regeneration in mixed forests. The study examined tree species composition, diversity and dynamics of natural regeneration in gaps under three contrasting forest stands at Křtiny in the Czech Republic. In spring 2013, experimental gap design begins, when semi-permanent 1 m2 circular sub-sampling plots along North-South-East-West transects were delineated under 6 selected natural canopy openings ≤ 20 m2. In winter 2013/14, these naturally originated openings were artificially enlarged to the current gap sizes ranging between 255 and 1149 m2 through group felling. Natural regeneration in gaps was measured four times: from the growing season before disturbance (BD) in 2013 to the next three consecutive growing seasons after disturbance in 2014 – 2016, respectively. Seven (7) new species with light demanding growth strategy that were previously not present at mother stands were occurring there during the first growing season after disturbance (FGS), yielding the highest taxa (14 species) and diversity (Shannon diversity index, H = 1.7) while BD attained the lowest (8 species; H = 0.9), respectively. Study site being part of Fagus sylvatica vegetation community and providing favorable natural conditions for the optimal growth of Picea abies significantly explains the regeneration dominance of these species in gap regeneration from BD until the third growing season after disturbance (TGS), respectively. Small scale gap-disturbance contributed to the higher regeneration densities of all studied species during FGS. However, drought, competition from other life forms, and browsing activities substantially caused a progressive decline in natural regeneration during three consecutive years after disturbance.

Short-lived legacies of Prunus serotina plant-soil feedbacks.

Plant-soil feedbacks (PSFs) are often involved in fundamental ecological processes such as plant succession and species coexistence. After a plant initiating PSFs dies, legacies of PSFs occurring as soil signatures that influence subsequent plants could persist for an unknown duration. Altered resource environments following plant death (especially light availability) could affect whether legacy effects manifest and persist. To evaluate PSFs and their legacies, we obtained soils from a chronosequence of Prunus serotina harvests. In a greenhouse experiment, we planted conspecific seedlings under two light levels in these soils of varying time since the influence of live Prunus serotina, and compared seed/seedling survival in soils from live trees, stumps, and surrounding forest matrix within each site and across the chronosequence. PSF legacies were measured as the difference between seedling performance in live tree and stump soils within a site. Negative PSF legacies of P. serotina were short-lived, lasting up to 0.5years after tree removal. These effects occurred under 5% but not 30% full sun. PSFs and their legacies manifested in seed/seedling survival, but not biomass. Though restricted to low light, short-lived legacies of P. serotina PSFs could have lasting impacts on plant community dynamics during post-disturbance regeneration by disfavoring P. serotina regeneration in small tree-fall gaps.

The effects of stump size and within-gap position on sprout non-structural carbohydrates concentrations and regeneration in forest gaps vary among species with different shade tolerances

BackgroundTo restore secondary forests (major forest resources worldwide), it is essential to accelerate the natural regeneration of dominant trees by altering micro-environments. Forest gaps are products of various disturbances, ranging from natural storms or wildfires to anthropogenic events like logging and slashing-and-burning, and sprouts of most tree species with non-structural carbohydrates (NSCs) storage can regenerate from stumps after gap formation. However, how the stump sprouts with diverse NSCs storages and stump sizes (i.e., diameters) adapt to various micro-environments of within-gap positions remains unclear. Therefore in this study, we monitored the stump sprout regeneration (density, survival, and growth) and NSCs concentrations of three dominant tree species with different shade tolerances and varying stump diameters at five within-gap positions for the first two consecutive years after gap formation.ResultsStump diameter was positively correlated with sprout density, growth, and survival of all three tree species, but insignificantly related with sprout NSCs concentrations at the early stage after gap formation. The effect of within-gap position on sprout NSCs concentrations was different among species. After an environmental adaptation of two growing seasons, the north of gap (higher light availability and lower soil moisture habitat) was the least conducive for shade-intolerant Quercus mongolica to accumulate leaf NSCs, and the east of gap (shadier and drier habitat) was conducive to increasing the leaf NSCs concentrations of shade-tolerant Tilia mandshurica.ConclusionsWithin-gap position significantly affected leaf NSCs concentrations of all three tree species, but most of the sprout growth, survival, and stem NSCs concentrations were independent of the various within-gap positions. Besides stump diameter, the NSCs stored in stump and root systems and the interspecific differences in shade tolerance also contributed more in sprout regeneration at the early stage (2 years) of gap formation. A prolonged monitoring (> 10 years) is needed to further examine the long-term effects of stump diameter and within-gap position on sprout regeneration. All of these findings could be applied to gap-based silviculture by promoting sprout regeneration of dominant tree species with different shade tolerances, which would help accelerate the restoration of temperate secondary forests.

Mind the gaps: Comparison of representative vs opportunistic assessment of tree regeneration in Central European beech forests

Tree regeneration (TR) in canopy gaps is a key process to understand how forest ecosystems might adapt to future environmental changes. Since successful TR is the result of a complex interplay of several stochastic events such as gap formation, seed production, ungulate pressure and diseases, some of the processes have been neglected or strongly reduced for model applications. Most empirical data on TR originate from (national) forest inventories based on statistically optimized designs for timber resource estimations and use geographically fixed permanent plots. Consequently, these representative samples record TR both in gaps as well as under closed canopy conditions. In this study, we compared TR in 63 representative plots of the Swiss National Forest Inventory (‘NFI sampling’), located in beech and silver fir-beech forests along an elevation gradient in the Jura Mts., with an opportunistic ‘gap sampling’ strategy targeting TR-clusters. We analyzed quantitative (e.g., number of individuals) and qualitative (e.g., species diversity, similarity to surrounding canopy) differences between the ‘NFI sampling’ and ‘gap sampling,’ as well as the environmental factors (e.g., gap size, cluster age, elevation) influencing the composition of TR in small canopy gaps. The ‘gap sampling’ recorded higher median TR densities (+62%, stems ha−1) and species richness (4.0 ± 2.1, mean ± sd) than the ‘NFI sampling’. More importantly, the ‘gap sampling’ provided much more consistent results among sites (i.e., 4x lower variance). Consequently, the ‘gap sampling’ revealed not only general but also species-specific patterns across TR size classes and along elevational gradients in contrast to the lack of patterns in the ‘NFI sampling’. Species richness decreased as TR grew taller, and the tallest TR size class (>130 cm) was almost solely comprised of the dominant species in the surrounding canopy (direct ingrowth). The high amount of direct ingrowth in TR indicates that small canopy gaps might offer only limited management potential to alter forests and that extensive disturbances or management at early TR stages (sapling) would be necessary to promote species better suited to the expected future site conditions. We raise the question of whether a forest monitoring that is statistically optimized for timber resource estimation is best suited to detect adaptation effects to quickly changing environments best visible at the regeneration stage, or whether forest inventory tools should be extended by methods that focus on disturbed parts of forests where changes and adaptations take place. We propose the implementation of an opportunistic gap-based monitoring of TR that essentially serves to identify the best forest management strategies (e.g. gap size, intervention intervals) in forests adapting to environmental change.

Abiotic and biotic determinants of height growth of Picea abies regeneration in small forest gaps in the Swiss Alps

In managed mountain forests, height growth of Norway spruce (Picea abies (L.) Karst) regeneration is a decisive factor for the gap-filling process, especially when the silvicultural goal is to provide continuous protection against natural hazards such as snow avalanches or rockfall. For the planning of management interventions, robust predictions on height growth of regeneration at the scale of forest gaps are thus needed. However, there is a lack of such data for trees of intermediate sizes, and existing studies fail to cover large environmental gradients. The goal of our study is to identify the key factors that influence height growth of Norway spruce regeneration in small gaps of spruce-dominated forests in the Swiss Alps. Furthermore, we assess whether there are site-specific differences of height growth or whether it follows a similar pattern along a large gradient of temperature (i.e., along elevation) and of water and nutrient availability (i.e., among different phytosociological site types) within the upper montane and subalpine vegetation belts. On 124 plots, >2′000 observations of annual height increments of Norway spruce regeneration (10 cm tree height to 12 cm stem diameter) in gaps were collected. Using linear mixed effects models and cross-validation for model selection, we identified the best variable combinations to predict annual height growth. Consistently across the entire gradient, the most important factors were 1) the positive effect of tree size, 2) the negative effect of competition by the surrounding stand, and 3) local topography. We found site-specific differences in height growth patterns such as gap size and therefore direct radiation being the most important competition measure in subalpine sites, as opposed to diffuse radiation in high montane sites. However, the pooled model for the entire environmental gradient allowed for predictions of regeneration height growth with similar explanatory power as the more specific models while containing comparable effect sizes. Furthermore, competition can be equally well expressed by metrics based on basal area measurements as by metrics derived from hemispherical photography. Based on these relatively simple models, accurate and robust predictions of the development of Norway spruce regeneration in gaps of managed mountain forests are possible.

Regeneration and succession: A 50-year gap dynamic in temperate secondary forests, Northeast China

Forest gaps are essential small-scale disturbances in forest succession. However, little attention has been paid to the long-term effects of gap dynamics on woody species regeneration and succession. We selected 20 medium and 25 large gaps representing a range of age classes (0–10, 10–20, 20–30, 30–40, and 40–50 years) from remote-sensing images obtained in 1964, 1976, 1986, 1993, 2003, and 2014 to examine long-term gap regeneration processes in a secondary forest in Northeast China. We conducted field surveys to determine the regeneration status, density index (DI), and richness index (RI) of selected gaps. The importance value (IV) of each woody species was calculated, and all species were classified by shade tolerance. The results showed that in gaps of 0–10 years, the DI and RI were mainly dominated by shade-intolerant species in large gaps and intermediate species in medium gaps. When gap age increased to 10–20 years, the greatest RI was observed in large gaps due to an increase in intermediate species, and the dominant species (IV > 0.1) changed from shrubs to trees (Acer mono). The DI and RI decreased over time in medium gaps, but A. mono remained the dominant species in these gaps. By 20–30 years after gap formation, large gaps showed decreased DI and RI, associated with the exclusion of shade-intolerant and intermediate species, and Tilia amurensis joined the dominant species class. In medium gaps, a decrease in DI was associated with self-thinning among intermediate shade-tolerant species. Once gap age exceeded 30 years, the DI, RI, and dominant species became stable in both large and medium gaps. Our findings indicated that natural gaps can improve the regeneration of late-successional species in secondary forests, especially at the first 30 years. However, there was a barrier in succession of the secondary forests to climax forests relying on natural gap regeneration due to the absence of key species such as Korean pine (Pinus koraiensis Sieb. et Zucc.). These results can provide a significant reference for close-to-nature management of temperate secondary forests in practice.

The composition and diversity of natural regeneration of tree species in gaps under different intensities of forest disturbance

The positive ecological interaction between gap formation and natural regeneration has been examined but little research has been carried out on the effects of gaps on natural regeneration in forests under different intensities of disturbance. This study evaluates the composition, diversity, regeneration density and abundance of natural regeneration of tree species in gaps in undisturbed, intermittently disturbed, and disturbed forest sites. Bia Tano Forest Reserve in Ghana was the study area and three gaps each were selected in the three forest site categories. Ten circular subsampling areas of 1 m2 were delineated at 2 m spacing along north, south, east, and west transects within individual gaps. Data on natural regeneration < 350 cm height were gathered. The results show that the intensity of disturbance was disproportional to gap size. Species diversity differed significantly between undisturbed and disturbed sites and, also between intermittently disturbed and disturbed sites for Simpson’s (1-D), Equitability (J), and Berger–Parker (B–P) indices. However, there was no significant difference among forest sites for Shannon diversity (H) and Margalef richness (MI) indices. Tree species composition on the sites differed. Regeneration density on the disturbed site was significantly higher than on the two other sites. Greater abundance and density of shade-dependent species on all sites identified them as opportunistic replacements of gap-dependent pioneers. Pioneer species giving way to shade tolerant species is a natural process, thus make them worst variant in gap regeneration.